They are 11, 8 and 6. It has not escaped my attention that in seven more years I will have three teenaged girls in the house.
Add a laminated contact list of people & #s for the girls that they can call or if a someone else finds the bag, allows them to contact the parents to notify them.
Under the whistle.
Keep the bag as light as possible because kids aren't very tolerant of carrying extra weight and are likely to put it down and leave it if it is a burden.
That's why I stuck with a single Mainstay bar. I want them to have something to eat (and all three of them really like Mainstay bars) but as you say I have to keep it reasonably light or they're going to ditch 'em.
My kids get very little candy or other sugary foods, but there's a sesame honey bar that they really like, I could add a couple of those without adding too much weight or making them act like a bunch of ferrets on speed. They also store well.
Stitch their name or label each item or add a little flower stick on to decorate things. Situation example: if they are lost/separated and use one of the items and then leave it behind, it helps trackers to know that it was theirs.
That is a really fine idea. I'll break out the label printer for this.
Also, having their name on it settles arguements between the girls as to what belongs to whom!

It gives them a sense of "this belongs to me, maybe I should take care of this!"
True.